A tunnel boring machine (“TBM”) is a tunnel excavation apparatus for forming tunnels in a variety of soil and rock strata. A conventional TBM produces a smooth circular tunnel wall, with minimal collateral disturbance. As discussed in U.S. Pat. No. 8,172,334, to Lindbergh et al, which is hereby incorporated by reference in its entirety, a conventional TBM typically includes a full face rotatably driven cutterhead that supports a plurality of cutter assemblies. Typically, a cutterhead may have 20, 50, 100, or more cutter assemblies rotatably mounted to the cutterhead.
A breakthrough that made TBMs efficient and reliable was the invention of the rotating head, developed by James S. Robbins. Initially, Robbins' TBM used rigid spikes rotating in a circular motion, but the spikes would frequently break. He discovered that by replacing these grinding spikes with longer lasting rotatable cutter assemblies this problem was significantly reduced. Since then, modern TBMs include rotatable cutter assemblies.
In operation, the cutter head is urged against a surface to be bored such that at least some of the cutter assemblies forcibly engage the surface. In some TBMs a plurality of opposing sets of hydraulic cylinders engage the tunnel walls to anchor the TBM, and separate thrust cylinders press the rotating cutterhead against the rock or ground surface. The cutterhead rotates about a longitudinal axis so that as the cutter assemblies are forcibly pressed against the surface they roll along the surface to fracture, loosen, grind, dislodge, and/or break materials from the surface.
As illustrated in Lindbergh et al., rotatable cutter assemblies are mounted in housings in the TBM cutterhead assembly such that the cutter ring extends forward from the face of the cutterhead assembly to engage the earthen rock wall. During operation of a TBM the cutterhead assembly is pressed with great force against the rock face, typically with hydraulic actuators, while the cutterhead is rotated about its axis. The outer cutter ring of the cutter assemblies produce local stresses that cause the surface of the wall to fracture and crumble. The fractured and loosened material is collected and removed to gradually form the tunnel.
Another illustrative tunnel boring machine is disclosed in U.S. Pat. No. 4,548,443, to Turner, which is hereby incorporated by reference. A main frame for a TBM is disclosed in U.S. Pat. No. RE 31511, to Spencer, which is hereby incorporated by reference in its entirety. A TBM with continuous forward propulsion is disclosed in U.S. Pat. No. 5,205,613, to Brown, which is hereby incorporated by reference. The TBM and a cutter disc assembly and sensor apparatus for a TBM disclosed in U.S. Pat. No. 8,172,334, to Lindbergh et al., provides a means for wireless monitoring the operation of the cutter assemblies.
The cutterhead assembly and the cutter assemblies are subjected to very high forces during tunnel boring operations. Once excavation of the tunnel is started, it is very difficult to repair or replace the cutter assemblies because the assemblies are difficult to access in situ, and the cutter assemblies are heavy, often weighing many hundreds of pounds. Tunnels are often at significant depths, with correspondingly high ambient pressures. Therefore, it is critical that the installation of the cutter assembly in the cutterhead be very secure and reliable, even under the extreme conditions associated with tunnel boring.
FIG. 1 herein shows an exploded view of a conventional cutter assembly housing for a tunnel boring machine, from Lindbergh et al. The cutter assembly 10, comprising a cutter ring 15 disposed on a hub 12 that is mounted for rotation about a shaft 13. Bearing assemblies (not shown) are mounted generally on the shaft 13 to provide for rotation of the hub 12 and cutter ring 15 about the shaft 13.
The conventional cutter housing shown in FIG. 1 comprises spaced-apart housing mounts 20L, 20R (sometimes referred to as mounting plates). Opposite ends of the shaft 13 are secured in the housing mounts 20L, 20R in L-shaped channels 21 (one visible) that are sized to receive the cutter assembly shaft 13. Typically the cutter assembly 10 is installed by positioning the opposite ends of the shaft 13 at the back of the housing mounts 20L, 20R to engage the long leg of the L-shaped channels 21. The cutter assembly 10 is slid along the long leg of the L-shaped channel 21 and then shifted laterally into the recess formed by the shorter leg of the L-shaped channels 21. The cutter housing secures the cutter assembly 10 to the housing mounts 20L, 20R with a pair of wedge-lock assemblies that engage respective ends of the shaft 13.
The wedge-lock assemblies each include a wedge 22, a clamp block 24, and an optional tubular sleeve 28 disposed therebetween. The wedge 22 is positioned to abut an angled face on the end of the shaft 13, and the clamp block 24 engages abutment surfaces 25 on the back end of the associated housing mount 20L, 20R. A bolt 23 extends through the wedge 22, the sleeve 28, and the clamp block 24, and is secured with two nuts 26 and a washer 27. As the bolt 23 is tensioned by torqueing the nuts 26 to a design specification, the wedge 22 locks the cutter assembly 10 in place.
In practice, this mounting has presented certain challenges and disadvantages. For example, the “wedge drop-down” (the cutter assembly 10 lateral shift into the shorter leg of the L-shaped channel 21) required to fit the wedge 22 into place requires space on the TBM cutterhead assembly can be challenging. In a typical installation the cutter assembly 10 drops about 4 inches into the housing pocket of channel 21 to enable installation of the wedge 22 to lock the cutter assembly 10 into positions via the bolt 23 that spans length of the housing mounts 20R, 20L.
In addition, the shallow angle on the wedge 22 is typically relied on to press the cutter assembly 10 laterally into the desired position in the channel 21. The more shallow the wedge angle or lower friction coefficient on the wedge 22, the more effective it is at holding the cutter assembly 10 in position via the mechanical advantage of the wedge 22.
The lateral shift makes it difficult to ensure that the cutter assembly shaft is securely supported in the housing. It will be appreciated by persons of skill in the art that if the shaft is not securely seated in the housing, for example, if any motion between the shaft and the housing develops, the high dynamic forces associated with the tunnel boring process will lead to rapid failure of the assembly. Situating the shaft in the lateral segment of the L-shaped channel makes it very difficult to detect if the shaft is properly seated, and does not provide for an effective mechanism for seating the shaft against both walls in the shifted portion of the channel.
Another disadvantage of this conventional design, that can be particularly prevalent when doing in-field maintenance, is that if dirt or other debris is unintentionally present in the L-shaped channel 21 when the wedge 22 is tightened to secure the cutter assembly 10, and the debris becomes dislodged during operation, the cutter assembly 10 may no longer be suitably secured, which can lead to serious damage to the cutter assembly 10 (and potentially the cutterhead), more rapid wear of the cutterhead 10, and more frequent maintenance requirements.
Also, removal of the cutter assembly 10 from the housing 20L, 20R is challenging, particularly for repair or replacement in the field, because the (heavy) cutter assembly 10 must usually be shifted laterally in the L-shaped channel 21 to align it with the long leg of the channel 21 prior to pulling the cutter assembly out.
There remains a need for improved and more reliable systems for mounting cutter assemblies to the cutterhead in tunnel boring machines.